MS RFC 93: UTF Grid Support

Author:Steve Lime
Author:Thomas Bonfort
Author:Daniel Morissette
Author:Mike Smith
Author:Francois Desjarlais
Version:MapServer 7.0

1. Overview

UTFGrid is a open solution for specifying raster interaction data. The goal being to provide highly scalable user interaction with raster maps. The objective of this RFC is to enable MapServer to produce UTFGrid output.

The UTFGrid encoding scheme encodes interactivity data for a raster tile in a space efficient manner. It is designed to be used in browsers, e.g. for displaying tooltips when hovering over certain features of a map tile.

Since slower browsers and machines can’t cope with rendering the actual polygons used to draw vectors on the map tile, we use a grid-based approach where we store the associated information for each pixel. UTFGrid uses JSON as a container format.

The full spec and interactive demos are available at

2. Proposed solution

This work would add the ability to write UTFGrids (JSON) as output from a normal map draw operation, that is, via CGI mode=map or WMS GetMap requests.

2.1 Rendering the UTFGrid

New UTFGrid renderer that would utilize the AGG library using an unsigned int32 canvas to encode feature IDs as their literal UTF-8 respresentation. Feature IDs (starting at 1) would be assigned sequentially as features are rendered and a reference to the feature’s key would be maintained as part of the rendering process. The list of feature keys will be searched first to avoid duplication. It is certainly reasonable to define a key for feature that respresents a thematic property, for example population age class, which would result far smaller output than keys that are unique to each feature. A layers key item is defined by the new UTFITEM attribute.

The UTFRenderer will draw points, lines and polygons based loosely on the underlying style definition.

Polygons: solid fill.
Lines: solid line with width. Patterns are not applied.
Points: rectangle based on the bounding box of the symbol will be used.

Labels can be rendered using their parent feature id (derived via features UTFITEM) where the labels bounding box is drawn to the map. It is also possible to choose the resolution of the grid. A larger resolution will make the grid smaller and therefore reduce its weight but also its precision. These are optionals capabilities which can be enabled through the output format:

  NAME "myUTFGrid"

It may be desirable NOT to remove duplicate feature id/key pairs since that process could be expensive depending on the number of features in the map. The format option duplicates=false can be used to skip this step. The resulting JSON file will be a bit larger.

Note: due to upcoming changes to text rendering, Truetype symbols and labels rendering aren’t implemented yet and should be added in the future.

2.2 Exposing Feature Properties

A new layer element (UTFDATA) is added to allow the creation of simple JSON templates using MapServer expression syntax identical to what is used with classObj->text class text syntax. UTFDATA is of type expressionObj. If UTFITEMs is set the UTFDATA expose those so that keys and data can be connected.

If a UTFITEM is not set the sequential id (based on rendering order) is being used.


UTFDATA "{\"id\":\"[fid]\", \"name\":\"[name]\", \"country\":\"[country_code]\", \"area\":\"[area]\"}"
UTFDATA "{\"name\":\"[name]\", \"country\":\"[country_code]\", \"area\":\"[area]\"}"

If no UTFDATA is provided no data beyond the UTFITEM values will be exposed and sometimes this is all that is required.

UTFGrids can be set on multiple layers but only one UTFGrid can be rendered at a time. If you wish to add multiples UTFGrid, with for example OpenLayers, you need to add new layer in the map for each of them.

2.3 Examples

This is how the output format should look like.

  NAME "utfgrid"
  MIMETYPE "application/json"
  EXTENSION "json"

And this is how the new UTFITEM and UTFDATA word should be added in the layer.

  NAME         basic
  DATA         ne_50m_admin_0_countries
  TYPE         POLYGON
  UTFITEM   "postal"
  UTFDATA   "{\"admin\":\"[admin]\"}"

Here is an image rendered as a JSON with UTFGrid renderer:

  "                            &&&&",
  "                       && &&&&&&",
  "                     &&&&&&&&&&&",
  "                   &&&&&&&&)))))",
  "                  &&& &&&)))))))",
  "                    &&&&))))))))",
  "                   &&&&)))))))))",
  "                 &&&&)))))))))))",
  "                &&&&&)))))))))))",
  "               &&&&&))))))))))))",
  "              &&&&&&))))))))))))",
  "            &&&&&&))))))))))))))",
  "         &&&&&&&&)))))))))))))))",
  "       &&&&&&&&&&)))))))))))))) ",
  "    &&&&&&&&&&&&&))))))))))))   ",
  "   &&&&&&&&&&&&&&)))))))))))    ",
  "   &&&&&&&&&&&&&&)))))))))))    ",
  "   &&&&&&&&&&&&&&)))))))))))    ",
  "   &&&&&&&&&&&&&&))))))))))))) !",
  "   &&&&&&&&&&&&&)))))))))))))))!",
  "    &&&&&&&&&&&&))))))))))))))  ",
  "    &&&&&&&&&& )))))))))))))))  ",
  "    &&&&&&&&   ))))))))))))     ",
  "       &&    $$ ))))))))))  ))) ",
  "          $$$$$$))))))))))) ))) ",
  "         $$$$$$  )))))))))) )   ",
  "         $$$$$$ $)))))))))      ",
  "         $$$$$$$$$))))          ",
  "         #$$$$$$$$))))$         ",
  "         ##$$$$$$$###    ''''''(",
  "          ###########'''''''''''",
  "    %%%%#############'''''''''''"

And now we get the following keys and datas if we use the UTFITEM with the postal code as the key :


And if we turn off the UTFITEM we obtain the following keys and datas:


It is also possible to only keeps the UTFITEM which returns :


3. Implementation Details


  • there are two new keywords (UTFDATA and UTFITEM) that exists at the layer-level
  • one new renderer (UTFGRID) will be made available

Known limitations of the initial implementation produced during GSoC:

  • Attribute values are not JSON-escaped, so the JSON output could be broken if the attribute values contain invalid characters. A JSON escaping function should be added for UTFGrid and would also be useful for general template support of JSON output.
  • Due to the upcoming changes to TrueType rendering, the renderTrueTypeSymbols method has not been implemented yet. All other types of symbols and labels are implemented.

4. Files affected

The following files will be modified/added by this RFC:

mapserver.h: Add new output format defines, extend layerObj, etc...
mapfile.c/mapfile.h: Add new keywords: UTFITEM and UTFDATA.
mapoutput.c: Add appropriate entries for new UTFGRID driver.
maputfgrid.cpp/maputfgrid.h: Implementation of the new UTFGrid renderer.
mapaggcommon.h: Common resources for AGG and UTFGrid drivers.

5. OpenLayers

There is a patch submitted to the OpenLayers development team allowing use of UTFGrid on a WMS service. It is required in order to use the MapServer implementation of UTFGrid with OpenLayers

You can find it at in the utfgridwms branch. See also pull request at

There is a working example in the OpenLayers library (exemples/utfgridwms.html).

6. MapScript

PHP MapScript will need get/set methods added to the layer class. Swig-based MapScript should need no changes.

7. Backwards Compatibility Issues

None anticipated.

8. Security implications

None anticipated.

9. Performance implications

No impacts on core performance are anticipated.

10. Documentation needs

The layer object documentation will need to be updated to reflect the new parameters. The output section will need a new UTFGrid HowTo page to talk about configuration options associated with the new output format.

11. Bug ID and references

12. Voting history

+1 from DanielM, ThomasB, PerryN, SteveW, MikeS, SteveL, YewondwossenA, TamasS and StephanM